The present invention relates to a coil and method for magnetizing an article, particularly an electromagnetic coil method and apparatus for magnetizing an article employing relatively high magnetic fields.
Electromagnetic coils have long been employed for creating the magnetic fields necessary to magnetize an article comprising a permanently magnetizable material. Such material has as one of its material properties a coercive force that must be reached or exceeded by action of the coil in order to permanently magnetize the material.
In typical practice, the magnetic fields required to be produced by the coil are of such strength that substantial physical stress is imposed on the coil. This results from the Lorentz force between the moving electron charges that form the electrical current in the coil and the magnetic field produced thereby. The Lorentz force produces a radial stress on the coil as well as a consequent shrinking thereof along the longitudinal axis. The Lorentz force is proportional to the product of the velocity of the charges and the magnitude of the magnetic field. The velocity of the charges is proportional to the current which is also proportional to the magnetic field, therefore, the Lorentz force is proportional to the square of the magnetic field the coil produces. Accordingly, as the magnetic field is increased, the Lorentz force becomes disproportionately larger and the stress on the coil in practice is often on the order of thousands of pounds per square inch.
The coil also produces heat in proportion to the time integral of the square of the current it carries. If uncontrolled, this heat may destroy electrical insulation on the coil or even melt or vaporize it. To carry off this heat, coiled tubes adapted to carry a coolant such as water are typically wrapped around the outside of the electric coil. In addition, to restrain the coil against the aforementioned radial stresses, the coil is typically wrapped circumferentially with a strong jacketing material, such as a glass or aramid fiber tape. However, the jacket material also tends to be thermally insulative. Therefore, if the jacketing material is placed between the electric coil and the coiled tubes, transfer of heat is made less effective. If, on the other hand, the jacketing material is placed outside both the electric coil and the coiled tubes, the coiled tubes are subjected to being crushed between the electric coil and the relatively unyielding jacketing. Providing for effective cooling in the presence of large mechanical stresses has generally meant that structures adapted for carrying coolant are constructed to be very strong. As an example, Schuster et al., U.S. Pat. No. 4,529,955 proposes machining cooling channels in a stiff washer-like member. This type of construction has the disadvantage of being relatively expensive.
Another problem encountered in the design of a coil for magnetizing an article results from the need to avoid inducing voltages and currents in the, typically, electrically conductive material of which the cooling coils are formed. The rate of change of flux induces electric currents to flow in closed, electrically conductive paths enclosing the flux. Where coiled tubes enclose changing flux, currents flowing in the tubes heat the tubes, thereby tending to defeat their capability to provide for effective cooling. Moreover, the coiled tubes are typically connected to an exterior water supply, such as a recirculating chiller or facility water supply, and induced voltages may couple to these facilities exposing persons to danger. Where coiled tubes are wound in a similar manner and orientation to the windings of the electric coils, maximum induced voltages and currents are produced.
To reduce the induced voltages, the cooling coils may be wrapped in one direction over a part of the coil and in the other direction over a geometrically symmetrical part of the coil, so that induced voltages oppose one another and cancel out. However, where there are remaining asymmetries in the magnetic properties of the flux path, such as those introduced by the part that is being magnetized as a result of asymmetric properties or of the placement of the part within the coil, cancellation of the induced voltages is not complete.
Turning to considerations of the electric coil, individual coils thereof are typically formed around a tubular form or bobbin. The material of which the electric coils are formed will typically be provided with a large cross-sectional area for conducting large currents with low resistance. However, where the coils are formed by bending straight material around the bobbin, the bending force required will be greater for a greater cross-sectional area. This is especially so for material that is substantially radially symmetric in cross-section, such as ordinary round wire, where the bending moment of inertia for the cross-section increases with increasing cross-sectional area.
Yet another problem encountered in the design of coils for magnetizing an article is that the field inside the coil is typically not uniform. The field varies radially in relation to distance away from the center axis of the coil, being less at the center for a uniformly wound coil of finite length. Where the field inside the coil is not uniform, a greater current must be carried by the electric coil in order for producing a greater field, to ensure that the field everywhere equals or exceeds that required to provide the necessary coercive force. To the extent that the field must be increased because of its nonuniformity, the problems described above are exacerbated.
Accordingly, there is a need for a coil for magnetizing an article that provides for improved cost and ease of manufacture of an electric coil for generating a large magnetic field, that provides for more effective removal of heat from the coil while maintaining sufficient strength in the coil to resist magnetically induced stresses and while reducing or eliminating induced voltages and eddy currents in electrically conductive cooling apparatus, and that decreases the current carrying requirements of the electric coil to achieve the required magnetic field.